There are many wireless communication applications in the prior art that use mobile communication units. One application area is radio frequency identification (RF ID) systems, which involve a large number of these mobile units. A mobile antenna is an antenna that is attached to an article capable of moving.
There is a desire to minimize the cost of these mobile units, particularly when, such as in RF ID, there are a large number of mobile units in a system. While a battery is a reliable way to supply power to these mobile units, there are significant manufacturing costs incurred in using this device. Furthermore, batteries dictate certain shapes of the mobile unit, occupy area on the mobile circuit, and reduce the mechanical flexibility of the mobile circuit.
Passive mobile circuits exist in the prior art that have no battery supplying power to the circuit. These passive mobile circuits use the antenna structure on the circuit to seize energy from an electromagnetic field received by the passive circuit. The antenna used the energy in the field to provide the passive circuit with the power (both energy and voltage) to operate. In addition, these antennas detect (field detection) the presence of the electromagnetic field and may re-radiate a signal.
Typical field probing antennas in the prior art are made of short (non-resonant) dipoles loaded with a diode that is connected to the signal processing equipment (part of the mobile circuit) via a pair of high-impedance leads. These field-probing antennas are designed to minimize the disturbance to the field being detected.
One fundamental problem with passive tags is that the range is limited by the voltage picked up by the tag antenna and rectified by the tag power conditioning circuits. The voltage must be high enough to run the tag electronics, and the voltage is generally the limiting factor in determining the distance from the base station antenna at which the tags may be used. Even active tags having a battery to run the tag electronics are limited in the voltage picked up by the tag antenna.